In a point to multipoint radio system, such as in satellite radio access systems or in cellular mobile communications systems, the resources (band width) to be assigned are limited as they must be shared between the different communications which are active at the same time. Different methods are applied in order to share said limited resources. CDMA, FDMA (Frequency Division Multiple Access) or TDMA (Time Division Multiple Access) are among some of the methods known in the art.
In CDMA, the bitrate of every digital communication, be it from a data source or from a voice source after voice coding, is sent to the air modulating a carrier and using a pre-assigned bandwidth. The carrier is modulated using known techniques such as PSK (Phase Shift Keying), using different amplitude values in different carrier phases, giving rise to the so-called “constellation size”. In CDMA systems, all active communications send their modulated signals in the same carrier. In order to allow radio terminals to recover their corresponding signals, each signal is multiplied by a code. The result of each multiplication process modulates the same carrier and the addition of all the modulated signals is then sent through the transmission channel. The main transmission channel effects are signal attenuation, and noise and interference addition. . At the receiver end, the signal is multiplied again by the same corresponding code used in the transmission. The codes used for different transmissions are orthogonal as is well known in the related art. In every system, a particular set of orthogonal codes is used. The code bitrate is faster than the user bitrate, so the signal power is spread and converted into an absolute value. The ratio between the chip rate of the spreading code and the user bit rate is known as “Processing Gain”.
In order to protect the information against errors, the user bitrate is passed through a forward error correction code (FEC) before spreading and transmission. The codes considered in the present invention can be of any sort, including concatenated codes, and, within this category they can be the concatenation with repetition codes. The added bits resulting from such multiplication allow the receiver to improve the Bit Error Rate (BER). The ratio between coded bits rate and user input bits rate is known as FEC rate (R). By way of example it may be said that R=½ means two output bits per user bit. Thus the more bits are added, or in other words the lower R is, the more the BER improvement can increase.
Rate efficiency is then defined as the total number of user bits transmitted in frequency (per hertz) and per time unit in accordance with the following relationship:Reff=(Nc×R×M)/GWhere:    Nc is the number of orthogonal codes used    R is FEC rate    M is constellation size    G is spreading gain.
It therefore turns out that the rate efficiency is directly proportional to the value of R, i.e. better efficiencies are obtained for higher values of R.
On the other hand, in a TDM/TDMA system, the channels are distinguished using different time intervals or slots. Sometimes, slot capacity is limited by the system design, such that the more bits are added in the convolutional, the less effective payload is sent in the channel. In this case, if the same FEC is used, the channels addressed which require less signal to noise ratio, waste payload capacity with the unnecessary extra FEC bits. In the applications considered in the present invention, the use of CDMA and TDMA systems at the same time is also contemplated.
One solution can be the use of slots of different lengths depending on the required FEC. However, this solution has the drawback of requiring the addition of a frame header (frame descriptor). In certain applications, particularly in the present design of a Skybridge (a known satellite system for wireless access), terminals try to detect and demodulate all the spreading codes. The terminal then discards the packets which are destined to other terminals and selects the packets sent for its own. In order to do this, the terminal uses a cyclic redundancy check (CRC). This code is added after dividing the packet payload by a fixed divider. The added CRC is the rest of the division process. In the receiving side, the operation is reproduced and the same CRC must be obtained. This is possible because all the packets and time slots have the same structure. Now, if we assume that time slots have different lengths or structures, it is necessary to indicate the position and structure of each packet. This can be done using a frame descriptor inserted in each frame. This descriptor provides all the information needed to process the signal. A frame descriptor has several drawbacks such as complexity, capacity waste (the descriptor must be transmitted with sufficient power in order that all terminals can detect it) and possibly latency increase.